Passivation of stainless steel



Patented July 30, 1940 UNITED STATES PATENT OFFICE PASSIVATION OF STAINLESS STEEL No Drawing. Application April 25, 1939,

- Serial No. 269,980

11 Claims.

This invention relates to a method of passivating stainless steel. to render the same proof against attack by sulphuric acid.

Stainless steel, while highly resistant to attack by sulphuric acid, is not proof thereagainst. Apparatus constructed from sheet stainless steel subjected to attack continuously by dilute sulphuric acid may be resistant to the extent that destructive action takes place at the rate of .0035 to .010 inch per month or lesser amounts. While such attack is relatively small, yet when thin strip material is so attacked from both sides, but a short time may elapse before complete destruction results. For example, a member 0.10" thick would be ruined in from five to fifteen months at the attack rate specified, if attacked from both sides.

The present invention is directed to a method of rendering such stainless steels proof against such attack, as distinguished from resistant to attack and an object of the invention is, therefore, to provide a method of treating stainless steel to render it proof against attack by sulphuric acid, and to provide such a method which may be inexpensive and practical to follow.

The above and other objects and features of the invention will appear more fully from the following description of a preferred process and the results obtainable thereby, although it is to be understood that for a definition of the limits of the invention reference to the appended claims should be had.

In order to render, for example, stainless steel having 18% chromium and 8% nickel or 18% chromium alone, (known as 18-8 and 18-0), the stainless steel is immersed in dilute sulphuric acid of a strength approximately 3% for a period of 25 hours approximately and at room temperature. During this period gas is generated by the acid attacking the surface of the stainless steel and small bubbles completely cover the surface of theorder in size of a pin point. The gas generated is hydrogen.

During the course of this treatment and in about 24 hours, a black smudge gradually forms on the steel surface and the acid solution takes on a light green tinge. At the end of this time the stainless steel may be removed from the acid and then be thoroughly washed in water While at the same time it may be scrubbed non-abrasively to remove the smudge therefrom. These steps produce a nice appearing surface, proof against attack by sulphuric acid and forms the subject matter disclosed and claimed in the copending application of Leiter and Winlock Serial No. 127,-

198, filed February 23, 1938, of which the application is a continuation in part.

It has now been discovered that after subject ing stainless steel to dilute sulphuric acid for-24 hours approximately, as above described, such 6 treatment may be terminated and the steel then can be exposed to air for drying without washing or, washed for a considerable period, during which some of the smudge may be washed away, and then dried.

Upon drying in airthe smudge turns brownish or rusty in color and the stainless steel after being so exposed, for example for two hours approximately, and on becoming dry, will then be found to be proof against subsequent attack by sulphuric acid.

If washing is resorted to, a longer period than air drying will be required, but similarly upon a sufiicient time such as 24 hours, it may then without drying be subjected to sulphuric acid and be proof against attack.

If preferred, the steel may be washed for a few minutes and then dried in air thus producing a skin surface proof against attack.

In each case, subsequent to the 24 hours exposure to dilute sulphuric acid and the smudge formation, it is necessary to expose the surface for oxidation. This may result from exposure in air, or exposure to oxygen and air usually entrained in the wash water. Failure to expose the steel for a suflicient time to develop the oxide coating before exposing the steel to sulphuric acid will result in a surface not proof against attack and gradual corrosion.

Stainless steel pacified according to this process, when subsequently exposed to sulphuric acid in any concentration (even as" high as but other than fuming or concentrated sulphuric, is proof against attack. No bubbles form on the surface as did during the smudge formation and no activity whatever is present. The surface will also be found to be proof against attack from sulphuric even at temperatures as high as F., although at such temperatures, if due to evaporation, the acid becomes concentrated (as high as 94%) or fuming, attack will commence.

It will thus appear that through the procedure set forth, stainless steel may be rendered proof against all concentrations of sulphuric acid, and without perceptibly injuring the steel. Such procedure is relatively simple and may be effected with a minimum of operations.

While several procedures have been described for producing the passivation of stainless steel against sulphuric acid, and specified periods of treatment set forth, it is to be understood that the invention is not limited thereto but may be practiced in various other ways. As many variations which do not depart from the spirit of the invention can be made as will be understood by those skilled in the art, reference will be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

l. The method of passivating stainless steel to render it proof against corrosion by an aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid until a smudge is produced on the surface of the metal, terminating the acid treatment, and thereafter exposing the surface of the metal to an oxidizing medium to produce an oxide protective surface proof against attack by sulphuric acid other than concentrated fuming sulphuric acid.

2. The method of passivating stainless steel to render it proof against corrosion by an aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid for approximately 24 hours until a smudge is produced on the surface of the metal, terminating the acid treatment, and thereafter exposing the surface of the metal to an oxidizing medium to produce an oxide protective surface proof against subsequent attack by sulphuric acid other than concentrated or fuming sulphuric.

3. The method of passivating stainless steel to render it proof against corrosion by an aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid for approximately 24 hours until a smudge is produced on the surface of the metal, terminating the acid treatment, and thereafter exposing the surface of the metal to air for several hours to produce an oxide protective surface proof against subsequent attack by sulphuric acid other than concentrated or fuming sulphuric.

4. The method of passivating stainless steel to render it proof against corrosion by an aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be: treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid for approximately 24 hours until a smudge is produced on the surface -of the metal, terminating the acid treatment,

and thereafter exposing the surface of the metal to a short water washing and drying in air to produce an oxide protective surface proof against subsequent attack by sulphuric acid other than concentrated or fuming sulphuric.

6. The method of passivating stainless steel to render it proof against corrosion by a dilute aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid until a smudge is produced on the surface of the metal, terminating the acid treatment, and thereafter exposing the surface of the metal to an oxidizing medium to produce an oxide protective surface proof against attack by dilute sulphuric acid.

7. The method of passivating stainless steel to render it proof against corrosion by a dilute aqueous solution of sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with an agent consisting of a dilute aqueous solution of sulphuric acid until a smudge is produced on the surface of the metal, terminating the acid treatment and removing the smudge.

8. The method of passivating stainless steel to render it proof against corrosion by dilute aqueous sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with dilute aqueous sulphuric acid at a strength of about three per cent, until a substantially uniformly distributed smudge is produced on the surface of the metal, terminating the acid treatment, and removing the smudge by non-abrasive water washing,

9. The method of passivating stainless steel to render it proof against corrosion by dilute aqueous sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with dilute aqueous sulphuric acid at a strength of about three per cent, until a grayish black smudge is produced on the surface of the metal, terminating the acid treatment, and while the steel is still wet removing the smudge by water washing and non-abrasive brushing.

10. The method of passivating stainless steel to render it proof against corrosion by dilute aqueous sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with dilute aqueous sulphuric acid at a strength of about three per cent at about room temperature and until a grayish black smudge is produced on the surface of the steel, and thereafter terminating the acid treatment and removing the smudge.

11. The method of passivating stainless steel to render it proof against corrosion by dilute aqueous sulphuric acid, which comprises subjecting the stainless steel to be treated to contact with dilute aqueous sulphuric acid at a strength of about three per cent, at substantially F. for about twenty-four hours thereby producing on the surface of the steel a grayish black smudge, and thereafter terminating the acid treatment, and removing the smudge by water washing and non-abrasive brushing. 

